1. Field of the Invention
The present invention relates to an image reading apparatus and method, and in particular, to an image reading apparatus and method, which form image information of an original illuminated with light coming from a light source on a solid-state image sensing element via an optical imaging system, and read the image.
2. Description of the Related Art
Conventionally, various image reading apparatuses, each of which forms image information of, e.g., an original, on a plurality of line sensors including solid-state image sensing elements such as CCDs, via an optical imaging system, and digitally reads monochrome or color image information on the basis of the output signals from the line sensors, have been proposed.
FIG. 5 is a schematic view of an optical system of a conventional color image reading apparatus.
Referring to FIG. 5, reference numeral 100 denotes a platen glass on which an original to be read is placed; 101, a linear light source for illuminating an original; and 102, a reflector for improving the irradiation efficiency.
Light reflected by an original (not shown) illuminated with light emitted from the linear light source 101 and reflected by the reflector 102 is guided to an optical imaging system 104 via mirrors 103-a, 103-b, and 103-c, and the optical imaging system 104 forms image information of the original on a solid-state image sensing element 105.
The mirror 103-a moves at a scanning speed of v in a sub-scanning direction A of image information of the original, and the mirrors 103-b and 103-c move at a speed of v/2 in synchronism with the movement of the mirror 103-a, thus reading the image information in two dimensions in combination with the line-up direction (main scanning direction) of line sensors in the solid-state image sensing element 105.
In this arrangement, the image information formed on the solid-state image sensing element 105 is converted into an electrical signal and output. The electrical signal is sent to an output device (not shown) to output image information as a printout, or is sent to a storage device or the like to store the image information. Thus, the image reading apparatus is used for various purposes.
As a light source for an image reading apparatus of this type, a halogen lamp is generally used. Since the halogen lamp has high luminance but causes considerable temperature rise in the apparatus resulting from heat it produces and consumes electric power of 200 to 300 W, power consumption required for the entire apparatus increases.
In recent years, to avoid such problems, a high-luminance fluorescent and xenon lamps have been developed, and are used as the light sources of image reading apparatuses.
In most of fluorescent and xenon lamps, a small quantity of mercury grains and several Torr of Ar, Kr, Xe gas, or the like are sealed in a linear hollow tube, which has a structure in which the inner wall of the tube is coated with various fluorescent materials, and electrodes are placed at the two ends of the tube to tightly seal the tube.
The fluorescent material coated on the inner wall of the tube is excited by ultraviolet rays radiated from mercury or various kinds of gases due to discharge from the electrodes, and visible light is emitted in accordance with the emission characteristics of the fluorescent material. An appropriate fluorescent material is selected in accordance with the spectral energy characteristics required for the intended light source.
Especially, a color image reading apparatus requires a light source having a broad wavelength range corresponding to, e.g., RGB. When a light source having especially high luminance is required, a plurality of colors of fluorescent materials are mixed and applied to the inner wall of the tube.
The quantity of light (emission intensity) emitted from the fluorescent or xenon lamp is normally controlled by pulse-width modulation that controls the ON period using a constant current value, in place of a method of controlling the starting voltage unlike the halogen lamp. This is because of the emission characteristics of the fluorescent or xenon lamp, i.e., the lamp emits light when a given current value is exceeded, and the method of controlling the emitted quantity of light by controlling the current value cannot assure a broad control range of the emitted quantity of light.
On the other hand, the following technique has also been proposed. That is, in some image reading apparatuses using a fluorescent or xenon lamp, the aforementioned light quantity control is omitted, and the gain of, e.g., an amplifier for electrically amplifying the output signal from a solid-state image sensing element in accordance with a decrease in quantity of light due to aging is variably set to obtain an appropriate signal output by changing the gain in correspondence with the decrease in light quantity. In such technique, however, the S/N ratio of the read signal may vary depending on the gain value.
The aforementioned prior art suffers the following problems.
An image reading apparatus using a light source such as a fluorescent or xenon lamp, which has a fluorescent material as an emission source, as described in the above prior art, normally uses the technique of controlling the emitted quantity of light by controlling the pulse width corresponding to the ON period while maintaining a constant current value to be supplied to the lamp.
FIG. 6 shows the control waveform for controlling the emitted light quantity from the light source. The abscissa in FIG. 6 plots time, and the ordinate plots the current value that controls the emitted light quantity from the light source.
An Hsync period along the abscissa indicates the time corresponding to one accumulation period of a solid-state image sensing element, i.e., a charge accumulation period in accordance with the quantity of light that hits a light-receiving section of the solid-state image sensing element, as normally used.
Upon executing normal pulse-width control, a control signal is output once per accumulation period in synchronism with the leading or trailing edge position of a trigger signal indicating the start of this accumulation period. In this fashion, the light quantity control is performed in synchronism with a signal corresponding to a trigger signal for one accumulation period, thereby removing noise in an image signal arising from beat produced by interference between the pulse-width control that controls the quantity of light and the accumulation period.
On the other hand, in relation to the fluorescent or xenon lamps that use a fluorescent material as an emission source, a white light source having emission characteristics over the broad wavelength range that covers the entire visible light range obtained by mixing and applying some different color fluorescent materials is normally used in an image reading apparatus for reading color image.
When such white light source is used, a problem is raised due to different afterglow or persistence characteristics unique to the individual color fluorescent materials. The afterglow characteristics are determined by the time of the fluorescent material excited by ultraviolet rays stays at high energy level, and normally diminish exponentially.
This phenomenon suggests that emission remains even after a current that controls emission of the light source is cut off instantaneously and, depending on the characteristics of the fluorescent material used, it is given by:
T=e(xcfx84xe2x88x921) 
where xcfx84 is the characteristics determined by the fluorescent material. When fluorescent materials corresponding to RGB are mixed and used like in the white light source used in the color image reading apparatus, the afterglow characteristics of the respective colors are different from each other.
In general, materials used as fluorescent materials are determined in terms of the emission wavelength characteristics and emission efficiency of the materials in the respective wavelength ranges, service life, and the like. For example, the following materials are generally used:
Blue: BaMg2Al16O27 
central wavelength=452 nm
T=2 xcexcsec
Red: Y2O3:Eu2+
central wavelength=611 nm
T=1.1 msec
Green: LaPO$:Ce, Tb
central wavelength=544 nm
T=2.6 msec
where T is the decay time of each material, i.e., the time required until the emission time reaches 1/e by decay. In this manner, due to different afterglow characteristics of the respective colors (Blue has especially shorter decay time), the centers of gravity of individual colors read position are different in the sub-scanning direction.
Such phenomenon will be explained using FIG. 6.
The abscissa of the graph shown in FIG. 6 plots time, and the ordinate plots the current amount for driving the fluorescent lamp and the emitted light quantity of the fluorescent lamp.
Normally, the light quantity control (also referred to as lighting control, hereinafter) of the fluorescent lamp is performed once per Hsync period corresponding to one accumulation period of a solid-state image sensing element, and the solid-state image sensing element accumulates a charge proportional to the quantity of incoming light. A lighting control period in FIG. 6 corresponds to the time in which a current for driving the fluorescent lamp is kept supplied by an amount proportional to lighting control duty, and the current during this period is switched ON and OFF in high frequency by a conventional method. When the time corresponding to the lighting control period has elapsed, the emitted light quantity decays. The decay characteristics of the emitted light quantity are determined by the following two factors. One factor is the decay factor of line spectrum emitted from the fluorescent lamp, and the other factor is the decay characteristics of the fluorescent materials mentioned above. One accumulation period which ordinarily corresponds to Hsync is on the order of several hundreds of xcexcsec, while the decay characteristics of line spectrum are 1 xcexcsec or less, resulting in nearly no influence. However, the decay characteristics of the fluorescent materials are on the order of msec, resulting in large influences. Hence, the decay characteristics of the emitted light quantity are determined by the sum total of the two different emitted light quantities, and the decay characteristics of individual emissions.
FIG. 6 also shows an example of the afterglow produced by the decay characteristics of R, G, and B colors.
In the fluorescent lamp which is lighted on to have a nearly constant quantity of light by nearly constant current during the lighting control period, the quantity of light corresponding to line spectrum decays instantaneously after the end of the lighting control period. This portion corresponds to L1 in FIG. 6, and afterglow is produced by the decay characteristics of each fluorescent material with respect to the quantity of light corresponding to L2 in FIG. 6. The afterglow characteristics of the respective colors have the following problems in the image reading apparatus. One accumulation period of the solid-state image sensing element serves as a reference time upon reading image information, and also serves as a reference read position upon reading in the sub-scanning direction. The pixel density upon reading image information is determined by the pixel size of the solid-state image sensing element in the main scanning direction, and corresponds to the moving distance upon image reading scanned by mirrors or the like in the sub-scanning direction.
Hence, the phenomenon that the centers of gravity of the emitted light quantities of the respective colors with respect to Hsync are different in position due to their afterglow characteristics may be considered similarly to which replaces the abscissa of the graph in FIG. 6 with position information. This means that the read position in the sub-scanning direction has different centers of gravity for respective colors.
The different centers of gravity of the respective colors with respect to the read position in the sub-scanning direction cause color misregistration upon reading in the sub-scanning direction, and deteriorate the performance of the image reading apparatus.
It is an object of the present invention to provide an image reading apparatus which controls the quantity of light emitted from a light source so as to eliminate color misregistration upon reading in the sub-scanning direction due to different afterglow characteristics of the respective colors of the light source that illuminates an original.
It is another object of the present invention to provide an image reading method which eliminates color misregistration upon reading in the sub-scanning direction due to different afterglow characteristics of the respective colors of the light source that illuminates an original.
It is still another object of the present invention to provide a storage medium which stores an image reading method for eliminating color misregistration upon reading in the sub-scanning direction due to different afterglow characteristics of the respective colors of the light source that illuminates an original.
The object of the present invention is achieved by an image reading apparatus of the present invention comprising an original table for placing an original to be read, illumination means, having different afterglow characteristics for respective color components, for illuminating the original placed on the original table, a solid-state image sensing element for converting a received optical signal into an electrical signal, and outputting the electrical signal, an optical system for guiding an optical signal emitted from the illumination means and reflected by the original to the solid-state image sensing element, image output means for performing predetermined processing of the electrical signal output from the solid-state image sensing element, and outputting the processed signal as an image signal, and light quantity control means for controlling a quantity of light emitted from the illumination means, the light quantity control means divisionally executing light quantity control of the illumination means a plurality of times within one accumulation period of the solid-state image sensing element.
With this arrangement, the illumination means is controlled to have a plurality of ON and OFF periods within one accumulation period of the solid-state image sensing element, and the influences of different afterglow characteristics for respective color components are distributed over one accumulation period.
Hence, color misregistration upon reading in the sub-scanning direction, which occurs due to different afterglow characteristics of the respective colors can be remarkably eliminated upon executing the light quantity control of the light source that illuminates an original.
In the image reading apparatus, the light quantity control means preferably controls the quantity of light emitted from the illumination means by setting a plurality of equal ON periods at equal intervals in one accumulation period of the solid-state image sensing element.
The light quantity control means preferably uses pulse-width modulation in the light quantity control.
The illumination means may comprise a single light source for simultaneously outputting light beams of a plurality of color components.
Alternatively, the illumination means may comprise a plurality of light sources for respectively outputting light beams of different color components, and may output light by sequentially lighting on the individual light sources.
The illumination means preferably comprises a fluorescent lamp.
In this case, the fluorescent lamp preferably has a mixture of a plurality of types of fluorescent materials having different afterglow characteristics.
Furthermore, the apparatus may further comprise a plurality of solid-state image sensing elements for respectively receiving optical signals of different color components.
The other object of the present invention is achieved by an image reading method of the present invention comprising the illumination step of illuminating an original to be read placed on a original table by illumination means having different afterglow characteristics for respective color components, the light quantity control step of controlling a quantity of light emitted from the illumination means, the light guide step of guiding an optical signal emitted from the illumination means and reflected by the original to a solid-state image sensing element via an optical system, the conversion step of converting the received optical signal into an electrical signal by the solid-state image sensing element, and the image output step of performing predetermined processing of the electrical signal output from the solid-state image sensing element, and outputting the processed signal as an image signal, wherein the light quantity control step includes the step of divisionally executing light quantity control of the illumination means a plurality of times within one accumulation period of the solid-state image sensing element, and the conversion step includes the step of reading out a charge accumulated on the solid-state image sensing element after the plurality of times of light quantity control, and converting the readout charge into an electrical signal.
The still other object of the present invention is achieved by a storage medium of the present invention that stores an image reading method which can be implemented by a computer, the image reading method comprising the illumination step of illuminating an original to be read placed on a original table by illumination means having different afterglow characteristics for respective color components, the light quantity control step of controlling a quantity of light emitted from the illumination means, the light guide step of guiding an optical signal emitted from the illumination means and reflected by the original to a solid-state image sensing element via an optical system, the conversion step of converting the received optical signal into an electrical signal by the solid-state image sensing element, and the image output step of performing predetermined processing of the electrical signal output from the solid-state image sensing element, and outputting the processed signal as an image signal, wherein the light quantity control step includes the step of divisionally executing light quantity control of the illumination means a plurality of times within one accumulation period of the solid-state image sensing element, and the conversion step includes the step of reading out a charge accumulated on the solid-state image sensing element after the plurality of times of light quantity control, and converting the readout charge into an electrical signal.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.